134 MEMOIBS OP THE NATIONAL ACADEMY OE SCIENCES. 



have normal, well-developed, large black eyes, apparently entirely similar to those of the allied 

 shallow-water species ; Sclerocrangon agassizii, Bythocaris gracilis, Heterocarpus oryx, Nematocar- 

 cinus ensiferus, and N. cursor have normal black eyes, a little smaller than the allied shallow-water 

 species; Ethusina abyssicola and Parapagurus pilosimanus have distinctly facetted black eyes, which 

 though very much smaller than iu most shallow- water species, are still fully as large and apparently 

 quite as perfect as iu those of some shallow-water species in which they are evidently sensitive to 

 ordinary changes of light. The eyes of the species of Glyphocrangon are very large, with the 

 facetted surface much larger than in the allied shallow- water species ; but they are borne on very 

 short stalks, with comparatively little mobility, and have dark purple instead of black pigments. 

 The eyes of Pontophilus dbyssi are lighter in color than those of the species of Glyphocrangon, but 

 are facetted and apparently have some of the normal visual elements. All the species of Munidopsis 

 and Pentacheles have peculiarly modified eyes, from which the normal visual elements are appar- 

 ently wanting. Of these twenty-one abyssal species eight are thus seen to Lave normal 'black eyes, 

 two have abnormally small eyes, and three have eyes with purplish or very light-colored pigment, 

 while eight have eyes of doubtful function. If we confine the examination to the five species 

 taken below 2,000 fathoms, we have one with well developed black eyes, two with abnormally 

 small black eyes, one with light-colored eyes, and one with eyes of doubtful function. 



"These facts and the comparison of the eyes and the color of the abyssal species with the blind 

 and colorless cave-dwelling Crustaceans certainly indicate some difference in the conditions as to 

 light iu caverns and in the abysses of the ocean, and make it appear probable, in spite of the objec- 

 tions of the physicists, that some kind of luminous vibrations do penetrate to depths exceeding 

 even 2,000 fathoms. The fact that, excluding shallow-water species, there is no definite relation 

 between the amount of the modification of the eyes and the depth which the species inhabit, many 

 of the species with the most highly modified eyes being inhabitants of much less than 1,000 

 fathoms, might at first be thought antagonistic to this view. But when we consider how vastly 

 greater the purity of the water must be in the deep ocean far from land than in the comparatively 

 shallow waters near the borders of the continents, and how much more transparent the waters of 

 the ocean abysses than the surface waters above, we can readily understand that there may usually 

 be as much light at 2,000 fathoms in mid -ocean as at 500, or even at 200, near a continental border. 

 These considerations also explain how the eyes of specimens of species like Parapagurus pilosimanus 

 coming from 2,220 fathoms are not perceptibly different from the eyes of specimens from 250 

 fathoms. 



"Although some abyssal species do have well-developed black eyes, there can be no question 

 that there is a tendency towards very radical modification or obliteration of the normal visual 

 organs in species inhabiting deep water. The simplest and most direct form of this tendency is 

 shown in the gradual reduction in the number of the visual elements, resulting in the obsolescence 

 and in some cases in final obliteration of the eye. The stages of such a process are well repre- 

 sented even among the adults of living species. The abyssal species with black eyes, referred to 

 in a previous paragraph, contain the first part of such a series, beginning with species like Oeryon 

 quinquedens and Lithodes agassizii and ending with Ethusina abyssicola, in which there are only a 

 few visual elements at the tips of the immobile eye-stalks. A still later stage is represented by 

 A. Milne Edwards's genus Cymonomus, in which the eye-stalks are immobile spiny rods taper- 

 ing to obtuse points, without visual elements or even (according to the description) a cornea. 

 Cymonomus is not known to be an abyssal genus, neither of the species having been recorded 

 from much below 700 fathoms, and is a good example of the fact already mentioned, that many 

 of the species with the most highly modified eyes are inhabitants of comparatively shallow water. 

 There are, however, several cases of closely allied species inhabiting different depths, where the 

 e3 r es of the deeper- water species are much the smaller ; for example: Sympagurus pictus, 164 to 

 264 and Parapagurus pilosimanus, 250 to 2,221 fathom sj Pontophilus gracilis, 225 to 458, and P. 

 alyssi, 1,917 to 2,221 fathoms; and Nematocarcinus cursor, 384 to 838, and N. ensiferus, 588 to 

 2,033 fathoms. 



"In a large number of deep-water and abyssal species the ocular pigment is dark purplish, 

 brownish, reddish, light purplish, light reddish, or even nearly colorless, while the number of 

 visual elements may be either very much less or very much greater than usual. The eyes of the 



